Fix out-of-order notification processing

[agentcooper]

I've noticed that the ACP client sometimes receives text chunk updates in the incorrect order.

I've added a failing integration test where the client connects to the server that sends numbers from 1 to 100 and the client verifies the order.

The fix changes notification (messages like session/update) processing from concurrent goroutine spawning to a serial queue.

[inercia]

I'm also seeing this problem.

[ThomasK33]

Thanks for working on this and for adding the integration coverage around ordering.

I think this is effectively a duplicate of #8, and it also reintroduces the same class of queue-handling issues we discussed in the earlier iterations there:

• notificationQueue is bounded (make(chan *anyMessage, 1000)), but enqueue in the read loop is blocking (c.notificationQueue <- &msgCopy).
• If that queue fills, receive() can block and stop reading further frames, including responses, which can cause in-flight requests to hang.
• There is no explicit overflow policy (and if we add “fallback to concurrent handling” to avoid blocking, that would again break ordering guarantees).

Given that #8 already has the full history, discussion, and related fixes, I’d prefer that we consolidate there and close this as a duplicate.